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asbestos shingle or other high-temperature insulating material around the tool as it is heated. Large tools may require an acetylene torch with a large tip. Figure 3 lists twO color spectrums: one for hardening steel, the other for tempering. Nobody would claim to be able to tell exact temperatures by observing colors, but the range is close enough for most small tools. Experience helps. The chart lists the glowing colors as seen in moderate light. In bright light or in vety dim light, different temperature/color relationships apply. Higher-than-necessary temperatures may distort thin sections, so be sure to direct the flame at broad surfaces, not at sharp edges. Brazed cutters, like the twO shown on p. 73, should be held at the brazed point with pliers while heating to lower the temperature there so that the braze will not melt. Ann ealing-If you want to modify an existing tool by filing or sawmg, the steel must first be softened by annealing. Heat the steel with the torch, hold it at a temperature of about 1450°F for a minute or so and then cool it slowly. If the steel is cooled too rapidly, it will not be softened. Test it with a file, and if the file won't cut easily, try again. For most complete softening, the steel should be held at the elevated temperatute for as much as an hour and then cooled in a container filled with ground soapstone, fine sand, ashes from which all carbon has been removed, or a similar insulating material that will keep air away and cause the steel to cool over a long period of time, perhaps 10 hours. Large tool shops have special ovens for this. For softening small tools, it is usually adequate to heat the steel to a light red for a minute or so, and then slowly reduce the heat by gradually withdrawing the flame. When no color can be seen in dim light, the temperature is below 1000°F and the tool can be set aside until cool enough to test with a file. You can now shape the tool. Hardening-Hardening is accomplished by heating the steel to above its critical temperature (the same temperature as for annealing) and then quickly cooling it, usually by plunging it into a liquid. This is called quenching. The rate of cooling affects the hardness, the depth of penetration of the hardness, and, especially in large tools, the possibility of cracks forming. The most . Fig. 3: Colors observed in heat-treating Glowing colors: Lowest red visible in light 890°F Faint yellow Light straw Medium straw Dark straw Purple Blue Pale blue 420°F Oxide 76 440° 460° Approximate hardness, Rockwell C scale 6 �� \ 5 490° / Fine Woodworking 58 530° 560° 600° / / 57.5 57 56 \ Steel glows in the temperature range required for annealing and hardening, and the colors progress as in the top table, providing a built-in approximate temperature gauge. The temperatures required for tempering are much lower, and can be judged by observing the colored oxides that form on polished steel when it's heated. Colors are listed as they appear in moderate light-bright or dim light will change the relationships, and you may find that other authorities have their own names for the colors. No matter, hardness is easy to test, and experience brings consistency. colors: Further reading-For more about modern toolmaking as done in industry, consult the Tool Engineers Handbook (American Society of Tool Engineers, published by McGraw-Hill). Some of the older tool engineering books, such as High-Speed Steel by O.M. Becker (published by McGraw-Hill in 19 10), discuss in greater detail the simpler, then more often used methods. These and other useful and interesting books can be found in the industrial-arts reference section of a large public library, or in the reference section of an engineering college library. 0 Howard Lawrence is a retired aerospace engineer whose avocation was house and small-boat building and who now makes period-style furniture. He is a member of the Society of Philadelphia Woodworkers. Dull red 10201160° Full cherry 1300° Light red 1550° Full yellow 17501830° Light yellow 1900° common quenching liquids are water and oil. Water cools the steel faster than oil, but because of this faster cooling, water is more likely to cause cracks and distortion. Cracking isn't often a problem with small tools, though. Oil-quenching gives a softer core to the tool, and thus a tougher tool. Any kind of oil can be used for quenching, as long as it is thin and does not become gummy. Kerosene was often used in the past. Common salt (NaCl) added to water, making brine, also reduces cracking. The quenching solution should be warm, 125°F, although for small parts the temperature is not critical. Just make sure there is enough solution to allow you to completely immerse the part without appreciably raising the liquid's temperature. When the steel glows at the desired temperature, plunge the part into the quenching solution, cutting edge first, aiming to wet both sides of the tool at the same time. Agitate the tool and keep it in the solution until the bubbling stops. The cutter will now be so hard and brittle that a smooth file won't cut it. In use, such an edge would soon chip. Tempering-When a hardened tool is reheated to a particular temperature, it is softened to a predictable degree, toughening it. You can control the temperature by heating the tool in a household oven, or you can estimate the temperature by observing the oxide colors. The degree of tempering is partially a preference of the person doing the tempering. If the tool is too hard, it will chip easily; toO soft, and it will dull too soon. Tools are usually drawn to between a light straw color (quite hard) and a dark blue (softer). A sharp file will JUSt barely Cut steel drawn to a light straw. I prefer a light straw for shaper blades, and have never had one chip. Polish the face of the tool with fine sandpaper or emery cloth. Holding the tool with pliers as far back from the cutting edge as possible, apply low heat from the propane torch well back from the cutting edge. The polished metal will show a sequence of colors as the flame heats it, starting with a light yellow. As this color moves toward the cutting edge, it will be followed by darker shades of yellow, straw colors, purple, blue, etc. When the desired color reaches the cutting edge, plunge the blade in water to prevent the edge from absorbing any more heat from the body of the blade. The plane iron in the photo on p. 73 shows what a typical tool will look like at this stage. When tempering, heat the steel slowly to ensure even pene- tration into the tool and to prevent the color progression from moving to the cutting edge so fast that it would be difficult to Stop it at the proper time. The desired final temperature is only about 450°F for straw, 560°F for blue. When the edge is properly tempered, a smooth file will be just barely able to cut it.